Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Discrete ratio transmissions are capable of transmitting power via various power flow paths, each associated with a different speed ratio. A particular power flow path is established by engaging particular shift elements, such as clutches or brakes. Shifting from one gear ratio to another involves changing which shift elements are engaged. In automatic discrete ratio transmissions, a controller determines which power flow path should be engaged and establishes the power flow path by controlling the torque capacity of each shift element. The controller typically uses a signal from at least one speed sensor to determine what transmission ratio is suitable for current conditions. During a shift between ratios, the controller typically must measure the progress of the shift in order to determine the desired torque capacity for on-coming and off-going shift elements. Determining the current speed ratio during a shift requires signals from at least two different speed sensors.
FIG. 1 shows a portion of a transmission control system having two speed sensors. Each speed sensor produces a signal from which controller 10 can determine the speed of a particular shaft in the transmission. For example, speed sensor 12 may be associated with the turbine shaft and speed sensor 14 may be associated with the output shaft. A tone wheel is fixed to the shaft. The tone wheel has a number of teeth that pass by a sensing element. A magnetic field is established in the hall sensor such that the magnitude of the magnetic flux is relatively high when a tooth is adjacent to the hall sensing element and relatively low when a gap between teeth is adjacent to the sensing element. Depending on the design of the hall sensor, a single hall element may be used with signal processing based on the hall voltage that is generated by the magnetic field. More often, a group of hall cells is used a difference in voltage between hall cells voltage response is measured and used for improved sensing capability.
Controller 10 interacts with speed sensor 12 via a supply wire 16 and a return wire 18. Controller 10 establishes a voltage difference between the wires. Speed sensor 12 has circuitry such that electrical current on the supply and return wires is varied depending upon the magnetic flux at the sensing element. Commonly, the signal is binary such that the current is at a low level when the magnetic flux is less than a threshold and is at a high level when the magnetic flux exceeds the threshold. By noting the amount of time that passes between current level changes, the controller can calculate the speed and, in some cases, the direction of the shaft. Similarly, controller 10 interacts with speed sensor 14 via a supply wire 20 and a return wire 22 to determine the speed of a second shaft. Various types of electromagnetic interference act upon the wires between the controller and the speeds sensors, which may cause the voltage supplied at the speed sensor end of a wire to differ from the voltage at the controller end of the same wire. However, by twisting corresponding supply and return wires over the majority of the their length, the interference sources effect each wire almost equally such that the voltage difference between them remains close to constant along their length.
FIG. 2 schematically illustrates a sensor circuit suitable for either speed sensor 12 or 14 of FIG. 1. The majority of the circuitry is implemented using Application Specific Integrated Circuit (ASIC) 24. ASIC 24 has a supply terminal 26 to which the supply wire is connected and a return terminal 28 to which the return wire is connected. The sensing element is a collection of one or more hall plates 30 placed within the magnetic field to be sensed. Electrical current is passed through each hall plate from 32 to 34. The magnitude of the electrical current is determined by the voltage difference. The hall plates produce a voltage at output port 36 that depends upon the magnetic flux and upon the magnitude of the electrical current. Since the objective is to measure the flux, it is important to carefully control the current by carefully controlling the voltage across the hall plates. Although twisting the wires helps limit voltage fluctuations due to interference, additional voltage regulation at the sensor may be utilized for consistency of operation. In a typical implementation, a capacitor 38 may be placed between the supply terminal 26 and the return terminal 28. This capacitor is used to smooth voltage fluctuations and to protect the ASIC from abnormal voltage spikes. Capacitor 38 is external to ASIC 24 in order to facilitate changing the capacitance for different applications and because large capacitors are typically impractical to implement on the silicon substrate. A voltage regulator circuit 40 dynamically controls the supply voltage needed to operate the Hall plates and the signal processing circuit. This voltage level is the difference between the electrical potential be 32 and return terminal 28. Signal processing circuitry 42 creates a voltage output on connection 44 which then operates the output stage 46. Specifically, for a binary sensor, the sensor draws a continuous current that is function of the operation of the voltage regulator, signal processing block and the current used to operate the hall plates. The output stage then draws an additional current of some amount when actuated to provide a change in current draw which is substantially higher than the variability in the other sources of current such that controller 10 can easily distinguish real signal changes from noise.